This invention relates to tonneau covers to be placed over the cargo carrier of a vehicle, such as the bed of a pick-up truck or the like. In particular, the invention relates to an arrangement of the cover structure to facilitate attaching the flexible sheet component of the cover to the frame component, and especially, to compensate for problems caused by the fact that the sheet is made of non-isotropic material, so described because it is stiffer in a certain direction than in the perpendicular direction.
The type of tonneau covers currently used for covering the bed of a pick-up truck includes two primary components: a sheet of fabric or other flexible, preferably waterproof, material, and a frame to which the perimeter of the sheet is removably attached. The frame comprises elongated members, referred to as rails, that are normally extruded of durable, lightweight material, such as aluminum. Three of the rails are joined together by corner members and are rigidly attached to the side walls and the front walls of the bed by clamps, bolts, or adhesive means to extend along the upper rims of those walls, as shown in our U.S. Pat. Nos. 4,639,033 and 5,058,652, as well as in U.S. Pat. Nos. 4,730,866 and 4,838,602 of Nett.
In addition, tonneau cover frames currently in use also include another rail that overlies the tailgate. This rear rail extends between corner members at the rear ends of the side rails and is rigidly but not permanently attached to those members. All of the rails can be separated from the corner members to which they are connected, as shown in Ser. No. 612,482, supra. Such tonneau frames may be referred to as perimeter frames since they correspond to the perimeter of the sheet. They also correspond in size and configuration to the perimeter of the truck bed.
Our U.S. Pat. No. 4,639,033 discloses a tonneau cover structure in which a fabric sheet is attached to the rails of a perimeter frame by extruded elastomeric strips, each of which is attached to the lower surface of the sheet facing the frame. The elastomeric strips are referred to as J strips because they have a generally J-shaped cross-sectional configuration, with a flat part and a part that is curled. The flat part is sewn flat against the sheet. The curled edge is parallel to and adjacent the respective edge of the sheet, curving downwardly and inwardly so that it can be attached to its respective frame rail by being hooked over an outwardly facing edge of the rail. The dimensions of the sheet and rail structure were so related that the curled portion of each of the strips has to be uncurled to some extent to force it over the edge of the rail, either to hook or release the sheet from that edge. In attaching a strip to one of the frame rails, it is possible to press downwardly on the sheet directly over the curl along the outer region of one of the frame rail edges with sufficient force to force that curled portion past the rail edge to which it is being attached, whereupon the curled edge suddenly uncoils back to its normal curvature and snaps into place behind the rail edge.
The resilience of the J strip accommodates oversized frames and contraction of the sheet in cold weather by allowing the curled edge of the J strip to unwind to some extent. However, each of these factors makes it more difficult to either attach a tonneau cover sheet to or to detach it from the frame than it would be to perform those procedures under more desirable conditions. There is a limit to how much change in length such unwinding can provide. In addition, the fact that the sheet is also non-isotropic, i.e., that it is stiffer in the direction in which it is pulled during its fabrication than it is in the transverse direction, adds to the difficulty of fastening it to the frame.
The material used to make tonneau cover sheets is supplied on rolls. The greater stiffness is in the longitudinal direction wound around the core. The width of the material obtained on such rolls is sufficient to form the width dimension of a tonneau sheet but not great enough to form the longitudinal dimension. As a result, when a piece of the material long enough to produce a tonneau cover is cut from a roll, the stiffer direction of the cover sheet formed from that piece will be oriented in the longitudinal direction on the truck bed.
U.S. Pat. No. 5,058,652, discloses a tonneau cover frame that includes an extruded frame rail affixed to the top rim of the front wall of a truck bed. The frame has two or more flanges, each of which could receive an extruded elastomeric strip fastener. Such a frame structure provides for specific increments of change in the overall length of the sheet material to be attached to it, but it does not accommodate continuous changes in length that take place as the temperature goes from one level to another.
Furthermore, if the sheet is attached to the frame by snap fasteners, as shown in our application Ser. No. 612,482 and in the Nett patents, or by other means that do not permit incremental change, or that makes such change difficult, it is desirable to keep the sheet stretched to the desired degree of tautness when the temperature changes or when other conditions occur that change the tautness of the fabric.